1. Field of the Invention
The present invention relates to a measuring arrangement for capacitive security fences having a plurality of electrodes for identifying the individual sub-capacities between two respective electrodes and the self-capacitance sub-capacity between a respective electrode and ground, in which alarm or interference criteria are derived in a central evaluation device from the identified capacitance values and alarm or interference reports are displayed.
2. Description of the Prior Art
In order to reliably secure an object, the surroundings are frequently secured, for example, with a security fence, in addition to building security. Unauthorized penetration is to be prevented, or at least made more difficult. In order to perceive unauthorized penetration, capacitive security fences are constructed which trigger an alarm given the approach or given the penetration of an intruder. Objects particularly in need of surveillance and subject to jeopardy, for example, nuclear power plants or military installations, require an extremely reliable ambient security system.
Such ambient security systems, particularly open-air systems, are subject to certain disruptive influences so that the susceptibility to disruption is relatively high and false alarms are frequently triggered. Ambient security systems must therefore be constructed such that the effects to be indicated reliably lead to an alarm, but disruptions do not. Usually, the interelectrode capacitances between transmission and reception electrodes are measured and the resulting difference of the interelectrode capacitance is evaluated by way of a differential bridge (for example as disclosed in the German Letters Pat. No. 1,220,298, fully incorporated herein by this reference). Symmetrically occurring environmental influences can thereby be eliminated. An alarm is derived from the resulting differential capacitance. An alarm is therefore triggered either due to a discontinuous change of capacitance of a specific size or due to a steady change of capacitance having a defined rate of change. These relatively simple alarm criteria, however, also produce low protection against interference. In order to be able to reliably perceive an intruder, the response sensitivity of such a system can be increased. This, however, also generally means that the susceptibility to interference also becomes greater, i.e. the immunity from interference is reduced.
It is likewise been proposed to respectively measure the individual interelectrode capacitances between the electrodes and/or the self-capacitances between an electrode and ground, to store and evaluate the time curves of the individual capacitance changes and to derive an alarm criterion therefrom, as disclosed in the German patent application No. P 31 10 352.9. Such a method, however, has the disadvantage that a multitude of involved transfer devices must be provided in the proximity of the electrodes and that relatively high potentials, for example, 100 volts, must be constantly connected to and disconnected from the individual electrodes. Moreover, only one interelectrode capacitance or, respectively, self-capacitance, can be measured at a time. This means a longer time until all capacitances are measured and are available for evaluation. Since the constant transfer causes certain transient problems, this time is further increased.